1. Field of the Invention
The invention relates to a device for metering and equalizing an ink layer on the surface of an ink applicator roller in the inking unit of a printing machine.
2. Description of the Related Art
Metering devices of this type, known, for example, from DE 32 25 982 A1, are used, in particular, for ink applicator rollers in inking units of offset printing machines, the surface of such an ink applicator roller usually being elastic, since the plate cylinders cooperating with the ink applicator roller has a hard surface. In order to set a thin and uniform ink-layer thickness on the ink applicator roller, it is necessary to remove part of the ink layer located on the roller surface, in order to restrict or completely prevent undesirable ghosting. When the image segments of the plate cylinder transfer ink directly onto paper, small paper fibres may adhere to the surface of the plate cylinder and are then transferred onto the ink applicator roller.
Tests were conducted in the past to use stripping blades as means for metering the printing ink on the surface of ink applicator rollers, but the soft elastic surface of these was sometimes damaged. On the other hand, particles transferred from the plate cylinder onto the ink applicator roller and, furthermore, also the dampening medium located on the ink applicator roller accumulate along that edge of the stripping blade which cooperates with the roller, and this may lead to the formation of undesirable stripes and streaks on the roller surface.
By contrast, the object on which the present invention is based is to provide a metering device of the type mentioned in the introduction, by means of which a uniform ink layer is achieved on the roller surface, without streaks and stripes being formed there.
When using a device having a front blunt doctor blade and a rear doctor blade, a splitting-off of dampening medium is first achieved with the front doctor blade, in which a large part of the disturbing dirt particles are picked up. Consequently, such particles cannot build up along the rear doctor blade. Dirt particles which have nevertheless passed the front doctor blade are transported away together with the ink wiped off by the rear doctor blade. These measures reliably prevent the formation of streaks and stripes. The rear doctor blade ensures, with its substantially smaller doctor blade edge radius, that the excess ink fraction is wiped off, leaving a residual ink layer of defined thickness and metering, with the result that a high degree of equalization of the residual ink layer can be achieved. The respective throw-on pressure of the two doctor blades onto the roller surface, said pressure depending inter alia on the printing parameters, such as, for example, the printing speed, must be set, in this case, in such a way that the effects described above can arise.
The return of wiped-off ink is normally carried out by means of a pump with a drive motor, this having an adverse effect on the space requirement of the inking unit and on the manufacturing costs.
By contrast, according to an embodiment of the invention, the metering device contains a chamber which is delimited in the circumferential direction of the roller by the front doctor blade and the rear doctor blade, in the radial direction of the roller by a chamber bottom and the roller surface and in the axial direction of the roller by two side walls, the ink wiped off by the downstream doctor blade being capable of being retained in the chamber and, generating excess pressure there, of being discharged outwards via one or more chamber orifices. Due to the excess pressure in the chamber, the ink can be led away from the roller surface, without additional energy being expended. Thus, since the components necessary for a pump arrangement become superfluous, a space-saving and cost-effective solution, which is also less susceptible to faults, is obtained.
With the given geometry, surface quality and material of the doctor blade and roller and, in particular, a given circumferential speed of the roller, the ink-layer thickness is generally set as a function of the force with which the doctor blade is pressed onto the roller surface.
According to a further embodiment, however, the chamber is connected to a pressure source, preferably via the chamber orifices which are formed by passage bores in the chamber bottom, in order to produce a residual ink layer of specific thickness as a function of the pressure prevailing in each case in the chamber. It is thereby possible to control the layer thickness in a simple way by a defined pressure being set in the chamber, without the force with which the doctor blade is pressed on the roller surface having to be changed.
According to another embodiment of the invention, the metering device comprises a duct which extends preferably parallel to and at a radial distance from the chamber and which forms a portion of an ink circuit in which pressurized ink can be conveyed, the duct being constantly connected to the chamber by means of the passage bores in the chamber bottom. The pressure prevailing in the duct and in the chamber is then capable of being controlled or regulated by means of a throttle which, as seen in the direction of flow, is preferably downstream of the duct and is of adjustable flow cross section. By means of such a controllable throttle in the return, a static pressure can be generated in the chamber, thus resulting in a uniform ink layer of defined thickness. When ink is fed at a regulated pressure to the chamber, ink can be applied directly to the roller by means of the metering device, so that an extremely short inking unit with a short setting-up time and with high dynamics is obtained.
According to a further embodiment, the front and/or the rear doctor blade is provided with an ink-repelling coating or with a combination of an ink-repelling and an ink-friendly coating, with the result that a wetting of the doctor blade edges with ink is largely avoided. In addition to the beneficial prevention of the formation of stripes, the outlay in cleaning terms is thereby reduced.
The metering device preferably has a shorter length than the roller and is designed to be movable relative to the latter, preferably in the form of traversing, oscillating or swinging movements running parallel to the roller axis. This contributes to equalizing the ink in the longitudinal direction of the roller and to the avoidance of streaks.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.